Journal of Energy Bioscience 2024, Vol.15, No.5, 314-325 http://bioscipublisher.com/index.php/jeb 317 nanotubes (CNTs) and carbon nanodots (CNDs), have shown significant promise due to their high surface area and excellent electrical conductivity (Scherbahn et al., 2014; Wu et al., 2017). For instance, vertically aligned carbon nanotubes (vaCNT) have been used to achieve a maximal power density of 122 µW/cm², demonstrating better power exhibition and stability compared to other materials (Scherbahn et al., 2014). Additionally, gold nanoparticles (AuNPs) and macroporous gold electrodes have been employed to enhance bioelectrocatalytic activity, resulting in significantly higher power densities (Deng et al., 2008). The use of composite materials, such as polypyrrole (PPy) combined with CNTs and Fe3O4, has also been shown to improve the bioelectrocatalysis of enzymes, further enhancing the performance of biofuel cells (Perveen et al., 2021). 4.2 Surface modification techniques to improve enzyme-electrode interactions Surface modification techniques are essential to improve enzyme-electrode interactions, which are critical for efficient electron transfer. One effective method is the use of conductive polymers, such as poly (3-aminobenzoic acid-co-2-methoxyaniline-5-sulfonic acid) (PABMSA), which can act as promoters for enzyme bioelectrocatalysis (Scherbahn et al., 2014). Another approach involves the use of electrostatic layer-by-layer (LbL) techniques to create multilayer biocatalyst immobilization on electrodes, significantly enhancing bioelectrocatalytic activity and power output (Deng et al., 2008). Additionally, 3D printing technology has been utilized to create high-porosity electrodes, which are then surface-modified to improve enzyme immobilization and electrocatalysis (Jayapiriya and Goel, 2020). These techniques help in achieving better electron transfer and stability, which are crucial for the long-term performance of biofuel cells. 4.3 Nanomaterial-based electrodes for enhanced performance Nanomaterials have been extensively studied for their potential to enhance the performance of biofuel cells. The use of nanomaterials, such as carbon nanotubes, graphene, and metal oxides, provides a close wiring for electron transfer between the biocatalyst and the electrode, thereby improving efficiency (Poulpiquet et al., 2014; Mishra et al., 2021). For example, carbon nanodots (CNDs) have been used as immobilizing matrices and electron relays, resulting in high open-circuit voltages and power densities in methanol/O2 biofuel cells (Wu et al., 2017). The incorporation of nanocomposites, such as PPy/Au/CNT@Fe3O4, has also been shown to significantly improve the surface area and electrical communication, leading to enhanced bioelectrocatalysis and higher current densities (Perveen et al., 2021). These advancements in nanomaterial-based electrodes are critical for developing more efficient and stable biofuel cells. 5 Fuel Selection and Utilization 5.1 Types of fuels used in enzyme-catalyzed biofuel cells Enzyme-catalyzed biofuel cells (EBCs) utilize a variety of fuels, primarily focusing on renewable and biologically derived substrates. Common fuels include simple sugars like glucose, which are oxidized by enzymes such as glucose oxidase or dehydrogenase (Yamamoto et al., 2013; Gross et al., 2017). Additionally, more complex substrates like starchy biomass can be directly utilized through the action of enzyme cascades, which break down the starch into simpler sugars that can then be oxidized (Yamamoto et al., 2013). This approach not only broadens the range of usable fuels but also enhances the overall efficiency of the biofuel cells by enabling the use of readily available and inexpensive substrates. 5.2 Substrate optimization for maximizing energy output Optimizing the substrate concentration and conditions is crucial for maximizing the energy output of EBCs. Studies have shown that the concentration of substrates, enzyme cofactors, and electron transfer mediators significantly impact the power density of the cells. For instance, an optimal concentration of lactate, NAD+, and CaCl2 was found to yield the highest power density in a lactate-based EBC (Jeon et al., 2008). Additionally, the pH and temperature of the reaction environment are critical parameters that need to be finely tuned to ensure maximum enzyme activity and stability (Cooney et al., 2008; Jeon et al., 2008). Advanced techniques such as response surface methodology (RSM) have been employed to systematically optimize these conditions, leading to significant improvements in the performance of EBCs.
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